1. Field of the Invention
This invention is in the field of medicinal chemistry. In particular, the invention relates to derivatives of gambogic acid and analogs, and the discovery that these compounds are activators of caspases and inducers of apoptosis. The invention also relates to the use of these compounds as therapeutically effective anti-cancer agents.
2. Description of Background Art
Organisms eliminate unwanted cells by a process variously known as regulated cell death, programmed cell death or apoptosis. Such cell death occurs as a normal aspect of animal development, as well as in tissue homeostasis and aging (Glucksmann, A., Biol. Rev. Cambridge Philos. Soc. 26:59–86 (1951); Glucksmann, A., Archives de Biologie 76:419–437 (1965); Ellis, et al., Dev. 112:591–603 (1991); Vaux, et al., Cell 76:777–779 (1994)). Apoptosis regulates cell number, facilitates morphogenesis, removes harmful or otherwise abnormal cells and eliminates cells that have already performed their function. Additionally, apoptosis occurs in response to various physiological stresses, such as hypoxia or ischemia (PCT published application WO96/20721).
There are a number of morphological changes shared by cells experiencing regulated cell death, including plasma and nuclear membrane blebbing, cell shrinkage (condensation of nucleoplasm and cytoplasm), organelle relocalization and compaction, chromatin condensation and production of apoptotic bodies (membrane-enclosed particles containing intracellular material) (Orrenius, S., J. Internal Medicine 237:529–536 (1995)).
Apoptosis is achieved through an endogenous mechanism of cellular suicide (Wyllie, A. H., in Cell Death in Biology and Pathology, Bowen and Lockshin, eds., Chapman and Hall (1981), pp. 9–34). A cell activates its internally-encoded suicide program as a result of either internal or external signals. The suicide program is executed through the activation of a carefully regulated genetic program (Wyllie, et al., Int. Rev. Cyt. 68:251 (1980); Ellis, et al., Ann. Rev. Cell Bio. 7:663 (1991)). Apoptotic cells and bodies are usually recognized and cleared by neighboring cells or macrophages before lysis. Because of this clearance mechanism, inflammation is not induced despite the clearance of great numbers of cells (Orrenius, S., J. Internal Medicine 237:529–536 (1995)).
It has been found that a group of proteases are a key element in apoptosis (see, e.g. Thornberry, Chemistry and Biology 5:R97-R103 (1998); Thornberry, British Med. Bull. 53:478–490 (1996)). Genetic studies in the nematode Caenorhabditis elegans revealed that apoptotic cell death involves at least 14 genes, 2 of which are the pro-apoptotic (death-promoting) ced (for cell death abnormal) genes, ced-3 and ced-4. CED-3 is homologous to interleukin 1 beta-converting enzyme, a cysteine protease, which is now called caspase-1. When these data were ultimately applied to mammals, and upon further extensive investigation, it was found that the mammalian apoptosis system appears to involve a cascade of caspases, or a system that behaves like a cascade of caspases. At present, the caspase family of cysteine proteases comprises 14 different members, and more may be discovered in the future. All known caspases are synthesized as zymogens that require cleavage at an aspartyl residue prior to forming the active enzyme. Thus, caspases are capable of activating other caspases, in the manner of an amplifying cascade.
Apoptosis and caspases are thought to be crucial in the development of cancer (Apoptosis and Cancer Chemotherapy, Hickman and Dive, eds., Humana Press (1999)). There is mounting evidence that cancer cells, while containing caspases, lack parts of the molecular machinery that activates the caspase cascade. This makes the cancer cells lose their capacity to undergo cellular suicide and the cells become immortal—they become cancerous. In the case of the apoptosis process, control points are known to exist that represent points for intervention leading to activation. These control points include the CED-9—BCL-like and CED-3—ICE-like gene family products, which are intrinsic proteins regulating the decision of a cell to survive or die and executing part of the cell death process itself, respectively (Schmitt, et al., Biochem. Cell. Biol. 75:301–314 (1997)). BCL-like proteins include BCL-xL and BAX-alpha, which appear to function upstream of caspase activation. BCL-xL appears to prevent activation of the apoptotic protease cascade, whereas BAX-alpha accelerates activation of the apoptotic protease cascade.
It has been shown that chemotherapeutic (anti-cancer) drugs can trigger cancer cells to undergo suicide by activating the dormant caspase cascade. This may be a crucial aspect of the mode of action of most, if not all, known anticancer drugs (Los et al., Blood, 90(8):3118–3129 (1997); Friesen, et al., Nat. Med. 2:574 (1996)). The mechanism of action of current antineoplastic drugs frequently involves an attack at specific phases of the cell cycle. In brief, the cell cycle refers to the stages through which cells normally progress during their lifetime. Normally, cells exist in a resting phase termed Go. During multiplication, cells progress to a stage in which DNA synthesis occurs, termed S. Later, cell division, or mitosis, occurs in a phase called M. Antineoplastic drugs such as cytosine arabinoside, hydroxyurea, 6-mercaptopurine, and methotrexate are S phase specific, whereas antineoplastic drugs such as vincristine, vinblastine, and paclitaxel are M phase specific. Many slow growing tumors, e.g. colon cancers, exist primarily in the Go phase, whereas rapidly proliferating normal tissues, e.g. bone marrow, exist primarily in the S or M phase. Thus, a drug like 6-mercaptopurine can cause bone marrow toxicity while remaining ineffective for a slow growing tumor. Further aspects of the chemotherapy of neoplastic diseases are known to those skilled in the art (see, e.g., Hardman, et al., eds., Goodman and Gilman's The Pharmacological Basis of Therapeutics, Ninth Edition, McGraw-Hill, New York (1996), pp. 1225–1287). Thus, it is clear that the possibility exists for the activation of the caspase cascade, although the exact mechanisms for doing so are not clear at this point. It is equally clear that insufficient activity of the caspase cascade and consequent apoptotic events are implicated in various types of cancer. The development of caspase cascade activators and inducers of apoptosis is a highly desirable goal in the development of therapeutically-effective antineoplastic agents. Moreover, since autoimmune disease and certain degenerative diseases also involve the proliferation of abnormal cells, therapeutic treatment for these diseases could also involve the enhancement of the apoptotic process through the administration of appropriate caspase cascade activators and inducers of apoptosis.
Gambogic acid was isolated from gamboge and the structure was deduced from the 1H NMR spectrum and by comparison with morellin, which also has the xanthone structure as that of gambogic acid (Ahmad, S. A., et al. J. Chem. Soc. (C) 772–779 (1966); Ollis, W. D., et al. Tetrahedron, 21:1453–1470 (1965)).

Asano J., et al., Phytochemistry, 41:815–820 (1996), reported the isolation of several xanthones, including gambogic acid from gamboge. They reported that gambogic acid is cytotoxic to both HeLa and HEL cells.
Lin, L.-J., et al., Magn. Reson. Chem. 31:340–347 (1993), reported the isolation of gambogic acid, as well as isogambogic acid and isomorellinol. All 3 compounds were reported to be cytotoxic against KB and KB-V1 cell lines.
WO00/44216 disclosed gambogic acid, analogs and derivatives as activators of caspases and inducers of apoptosis.